The present invention relates generally to an auxiliary power unit (APU) having an integral firebox. More particularly this invention relates to an auxiliary power unit that can be retrofitted, installed, or removed as a single unit.
Many modern commercial and business aircraft are equipped with airborne auxiliary power units (APUs). The APU provides electrical and pneumatic power to the aircraft for such tasks as environmental control, lighting, powering electronics, main engine starting, etc. Because of the size and weight of an APU, smaller aircraft have not been able to take advantage of the benefits and convenience of an airborne APU.
Two aspects of an airborne APU installation, which are typically high in development and certification costs, are (1) the need for a redundant or damage tolerant mount systems, and (2) the need for a dedicated fire zone within the aircraft for the APU. To address the need for a redundant or damage tolerant mount system, airborne APU installations typically utilize a complex network of struts or links and/or extensive and costly stress analyses. Complying with fire zone requirements is usually accomplished by dedicating a fireproof zone for the APU installation in the aircraft. Alternatively, some aircraft manufacturers choose to install the APU in the aircraft within a fireproof enclosure, which may make the installation relatively complex and heavy, when compared with the airborne APU that is fully integrated with the aircraft.
U.S. Pat. No. 6,308,915 discloses an “integral tailcone and power assembly.” This patent discloses a tailcone and power assembly mountable to the body of an aircraft using a height adjustable dolly. In the '915 patent the auxiliary power assembly is bolted to the aircraft. The '915 patent does not disclose an APU with an integral firebox that is structurally created as a single integral unit. FIG. 3 of the '915 patent shows a firewall 17 that separates the turbine engine from the fuselage. The '915 patent does not disclose an enclosure and turbomachine air intake duct structurally combined and secured to the APU to form an integral unit with the APU.
U.S. Pat. No. 6,581,874 discloses a “mounting assembly for an aircraft auxiliary power unit and method.” The '874 patent discloses a suspension system to support an APU within a tailcone section for removal and installation of the APU. Further, the '874 patent does not disclose an integral firebox. FIG. 6 of the '874 patent discloses rails and beams 50 to mount a generator 34 to the plane. The '874 patent does not disclose a firebox or firewall.
Therefore, there is a need to incorporate into the design of the general aircraft a redundant/damage tolerant mount system and an integral fireproof enclosure for an APU, referred to herein as an enclosure. There is a need to reduce the development costs associated with the mount system and the enclosure, as well as a need to reduce the weight of the enclosure. Further, there is a need to create an affordable means to utilize APU's with small aircraft. Additionally, there is a need to retrofit existing aircraft with APUs having an integral firebox.